CHAPTER VI 

 THE WORK OF SNOW AND ICE 



We have seen that the atmosphere, the ground-water, and the 

 waters on the surface of the land bring about important changes 

 in its configuration. Ice in its various forms is still another agent 

 of change. 



Ice beneath the surface. The wedge-work of ice in the crevices 

 of rock has already been mentioned (p. 103). When the great 

 areas where water freezes during some part of the year are con- 

 sidered, it is clear that the aggregate effect of the freezing of water 

 in the pores and crevices of rock must be great in long periods of 

 time. The water which freezes in the soil also has some effect on 

 the surface. This is shown by the disturbance of the walls of 

 buildings if their foundations do not go below the depth of freezing, 

 and by the working up of stones and bowlders through the soil of 

 the fields, as freezing and thawing succeed each other. The frozen 

 water in the soil makes it solid, and temporarily retards or prevents 

 surface erosion. 



Snow is more wide-spread than any other form of ice, but the 

 ice of lakes and rivers is so wide-spread as to be well known. Ice 

 on high mountains and in high latitudes is less familiar. 



Ice on lakes and ponds. Since fresh water is densest at 39 

 Fahr., ice does not commonly form on the surface of a lake until 

 the temperature from top to bottom is reduced to this point. 

 Cooled below 39, the surface water fails to sink, and reduced to 

 32, it freezes. If the lake is small and shallow, it is likely to freeze 

 over completely in any region where the temperature is notably 

 below 32 for any considerable period of time. It is under these 

 circumstances that the lake ice becomes most effective. 



Let us suppose a lake in temperate latitudes, where the range 

 of winter temperature is considerable, to be frozen over when the 



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